Dr.Meter Moisture Sensor Meter, Soil Water Monitor, Hydrometer for Gardening, Farming, Indoor/Outdoor Use
From the product page on Amazon.com:
List Price: $29.99
Price: $8.99 & FREE Shipping on orders over $35
UPDATE 25 June 2016 (rating change):
After more usage of this device, I have to downgrade my rating. After using it for several months it stopped working. However, it may have been my fault for leaving it in the soil for extended periods instead of just to test. The probe's end because corroded and that probably stopped it from working.
I love finding a product that performs its purpose properly and with zero friction factor. We took this meter out of the box, inserted it into a planter, and it immediately responded to inform us of the current moisture content. We moved it around to other plants and got similar immediate readings. Some plants like to be moist, our Bougainvillea prefers to be stressed. Now we can know. This takes away the guess work for watering our plants. Love it.